In recent years, in the automobile field, high-tensile steel sheets have been increasingly used in order to reduce the weight of a vehicle and improve safety against collision.
Furthermore, the level of strength of high-tensile steel sheets has increased year by year, and for example, hot-stamped members having tensile strength of 1500 MPa or higher have been practically used. The hot-stamped member as used herein is a member obtained by applying press working in a state where a steel sheet is heated to approximately 900° C. to be softened, and at the same time, quenching and strengthening the steel sheet using a cooling effect (contact cooling) due to contact with a die, thereby achieving the tensile strength of a 1500 MPa class as described above and favorable dimensional accuracy.
Furthermore, for example, in the case of assembling a vehicle body, a resistance spot welding is frequently used in which two or more steel sheet members formed by steel sheets are overlapped, and energization is applied while pressure is being applied with electrodes.
With this resistance spot welding, a melted and solidified portion having an ellipse shape, in other words, a nugget is formed in the overlapped portion through energization and heating, whereby it is possible to join the plural steel sheet members.
For example, FIG. 1 is a diagram schematically illustrating distribution of hardness in a spot-welded portion 10 in the case where conventional energization conditions are applied to two transformation-induced plasticity (TRIP) members S11 and S12.
More specifically, (a) in FIG. 1 is a sectional view schematically illustrating the vicinity of the spot-welded portion 10 in which the vertical direction on the paper is set to the thickness direction (in other words, a direction in which pressure is applied with the electrodes) of the TRIP members S11 and S12. Note that, in the following descriptions in the specification of the present application, a diagram illustrating a cross-section of two overlapped members when viewed in a similar manner to that in (a) in FIG. 1 is also referred to as a “sectional view illustrating a/the spot-welded portion.”
Furthermore, (b) in FIG. 1 is a graph schematically illustrating distribution of Vickers hardness so as to correspond to (a) in FIG. 1.
A molten metal generated through resistance spot welding is cooled at a high cooling rate, and hence, martensite is more likely to form in a nugget 12. As a result, the nugget 12 has a structure harder than a base metal portion. Note that, in the case where the strength of the base metal is high, the carbon equivalent is generally high, so that Vickers hardness of the nugget is high.
As illustrated in FIG. 1, the spot-welded portion 10 includes the nugget 12 and a HAZ 14. The HAZ 14 includes a HAZ hardened portion 14H located close to the nugget 12, and a HAZ softening zone 14T formed in the vicinity of the HAZ hardened portion 14H. Furthermore, the softest zone 14L in HAZ exists at an inner peripheral edge of the HAZ softening zone 14T.
The quality of the spot-welded portion is often evaluated on the basis of a tensile shear strength and a cross-tension strength (the strength of joint in a peel direction), and it is known that the tensile shear strength increases with an increase in the strength of the base metal.
However, in the case where the base metal has a tensile strength higher than a 780 MPa class, the peel strength, typified by the cross-tension strength, tends to decrease with an increase in strength of the base metal.
Below, a cross-tension test based on JIS Z3137 (1999), which is designed for measuring the cross-tension strength, will be schematically described with reference to FIG. 2A.
As illustrated in FIG. 2A, in the cross-tension test, two test pieces S21 and S22 formed by steel sheets are orthogonally arranged, and are joined by forming the spot-welded portion 10 including the nugget 12 through resistance spot welding.
Then, the test pieces S21 and S22 are pulled in a direction in which they are peeled, and the peel strength is measured until the spot-welded portion 10 is fractured.
A fracture mode with the cross-tension test can be divided into the following:    (a) interface fracture in which an interface between sheets in the nugget fractures;    (b) partial plug fracture in which, as illustrated in FIG. 2B, a crack propagates within the nugget 12 (inner side than a nugget end 12E) and then, fracture advances in the thickness direction; and    (c) plug fracture in which, as illustrated in FIG. 2C, the nugget 12 does not break, and the outer peripheral portion of the nugget 12 fractures in the thickness direction.
FIG. 2D is a diagram illustrating an example of a correlation between a base metal tensile strength and a cross-tension strength.
In FIG. 2D, “black dots” represent the plug fracture, and “blank circles” represent the partial plug fracture.
As illustrated in FIG. 2D, the cross-tension strength is approximately 9 kN in the case of a hot-stamped member of a 1500 MPa class (a steel sheet member obtained by hot-stamping a steel sheet for hot-stamping whose tensile strength becomes a 1500 MPa class by being hot-stamped), and is approximately 4 kN in the case of a hot-stamping member of an 1800 MPa class (a steel sheet member obtained by hot-stamping a steel sheet for hot-stamping whose tensile strength becomes an 1800 MPa class by being hot-stamped).
On the other hand, the cross-tension strength of a high-strength steel sheet of a 980 MPa class or lower falls in the range of approximately 8 kN to 14 kN.
In other words, the cross-tension strength of the hot-stamped member of a 1500 MPa class or higher is significantly lower than that of the high-strength steel sheet of 980 MPa class or lower.
Furthermore, as for the fracture mode through the cross-tension test, the high-strength steel sheet of a 980 MPa class or lower is fractured mainly in relation to the plug fracture in which the outside of the nugget 12 fractures, whereas the hot-stamp member of a 1500 MPa class or the hot-stamped member of an 1800 MPa class is fractured mainly in relation to the partial plug fracture.
This shows that, in the case of the hot-stamped member of a 1500 MPa class or higher, a crack is more likely to occur in the nugget because the toughness is small in the nugget.
As described above, in the case of the spot welding of the high-strength steel sheet, it is considered that the peel strength reduces mainly because the toughness reduces with an increase in hardness of the nugget, and thus, fracture (partial plug fracture) is more likely to occur in the nugget.
In general, with an increase in the diameter of the nugget, the fracture mode is more likely to be the plug fracture rather than the partial plug fracture, and the strength of the spot-welded portion increases.
Thus, in order to improve the peel strength of the spot-welded portion of the high-tensile steel sheet, it is effective, for example, to increase the diameter of the nugget.
However, in the case where the high-tensile steel sheet is subjected to resistance spot welding, spattering of molten steel called splash is more likely to occur as compared with a case where mild steel is subjected to resistance spot welding, possibly making it difficult to increase the diameter of the nugget.
In order to suppress the occurrence of splash, it is effective, for example, to increase the compression force with the electrodes. However, there is a restriction resulting from equipment such as a limitation of a welding gun in terms of stiffness.
Furthermore, it can be considered that, by increasing the number of spots in spot welding, it is possible to reduce the load stress per spot in spot welding. However, deterioration in productivity is inevitable.
Furthermore, if the distance between spots in spot welding is reduced, electric current is diverted to the spot-welded portions that have been already formed, causing a problem in which nuggets cannot be formed in a stable manner.
In other words, a desirable technique is one that can improve the strength of an overlap-welded member with resistance spot welding without changing the diameter of the nugget from the conventional one.
As for the technique described above, a subsequent energization method is disclosed in which a nugget is formed with main energization, and after the nugget is cooled, energization is performed again (see, for example, Non-Patent Document 1).
With the subsequent energization method, as illustrated, for example, in FIG. 3, in a state where a predetermined compression force is applied with electrodes in resistance spot welding,    (A) a nugget is formed by applying first energization (main energization) under conventional normal conditions;    (B) a predetermined suspension time is set to cool until martensite is formed in the vicinity of the nugget; and    (C) second energization (subsequent energization) is applied, thereby tempering the martensite.
With the subsequent energization method as described above, each heat-affected zone (hereinafter, referred to as a HAZ) of the nugget and the spot-welded portion is tempered, whereby toughness is improved. Furthermore, the HAZ is softened and is easily deformed, whereby stress in a nugget end portion area is alleviated at the time of peeling. Thus, it is considered that the peel strength can be improved.
With the resistance spot welding employing the subsequent energization, after the nugget is formed through the main energization, the molten metal is rapidly cooled through an Ms point to an Mf point or lower, and martensite is formed.
The martensite thus formed becomes tempered martensite by controlling the electric current conditions and the like used in the subsequent energization to adjust a heat-inputted amount so as to raise temperatures to fall in an appropriate temperature range (in other words, not less than approximately 550 to 600° C. and not more than an Ac1 point as illustrated in FIG. 3) in which tempering is possible, and being cooled after the subsequent energization is completed.
FIG. 4 is a diagram schematically illustrating distribution of hardness in a spot-welded portion 10 after the spot-welded portion 10 is formed by overlapping test pieces S31 and S32, which are dual phase (DP) members or TRIP members, under normal conditions used in the conventional resistance spot welding illustrated in FIG. 3, and applying the subsequent energization.
More specifically, (a) in FIG. 4 is a sectional view illustrating a spot-welded portion, and (b) in FIG. 4 is a graph schematically showing distribution of Vickers hardness in which each position corresponds to that in (a) in FIG. 4.
In the case where the overlapped portion is welded through resistance spot welding using the subsequent energization as illustrated in FIG. 3, the spot-welded portion 10 is first formed through main energization.
At this point in time, as illustrated in (b) in FIG. 1, the spot-welded portion 10 includes the nugget 12 and the HAZ 14, and the HAZ 14 includes a HAZ hardened portion 14H proximate to the nugget 12, and a HAZ softening zone 14T formed in the vicinity of the HAZ hardened portion 14H. Furthermore, the softest zone 14L in HAZ exists at an inner peripheral edge of the HAZ softening zone 14T.
Then, by applying the subsequent energization to the spot-welded portion 10, the nugget 12 and the HAZ hardened portion 14H are tempered as illustrated in FIG. 4, and the hardness of the nugget 12 and the HAZ hardened portion 14H is decreased.
However, hard portions 14P locally remain in the HAZ hardened portion 14H. Thus, at the time of peeling, the hard portions in the HAZ 14 are not deformed, and deformation concentrates on the vicinity of the nugget end 12E. As a result, stress concentration on the nugget end 12E is not sufficiently improved.
Furthermore, FIG. 5 is a diagram schematically illustrating changes of a HAZ 14 in a spot-welded portion 10 in the case where resistance spot welding according to conventional normal conditions is applied to test pieces S41 and S42, which are hot-stamped member, to form the spot-welded portion 10, and the spot-welded portion 10 is subjected to subsequent energization.
More specifically, (a) in FIG. 5 is a sectional view illustrating a spot-welded portion including the nugget 12 formed through single energization applied to the test pieces S41 and S42, and (b) in FIG. 5 is a graph schematically showing distribution of Vickers hardness in which each position corresponds to that in (a) in FIG. 5.
Furthermore, (c) in FIG. 5 is a sectional view illustrating a spot-welded portion including the nugget 12 after the subsequent energization, and (d) in FIG. 5 is a graph schematically showing distribution of Vickers hardness in which each position corresponds to that in (c) in FIG. 5.
It should be noted that the long dashed double-short dashed line illustrated in (d) in FIG. 5 illustrates the distribution of Vickers hardness after the main energization and before the subsequent energization.
In the case where the subsequent energization is performed under appropriate conditions, a large area including the nugget 12 and the HAZ hardened portion 14H is tempered as illustrated in (d) in FIG. 5. However, tempering cannot be sufficiently performed between the nugget end 12E and the softest zone 14L in HAZ, and portions 14P having high Vickers hardness locally remain.
In other words, an effect of improving toughness through tempering cannot be sufficiently obtained, and hence, it is not easy to sufficiently secure peel strength of the spot-welded portion 10.
Furthermore, in the case where heat inputted is excessive during the subsequent energization, the HAZ hardened portion 14H is tempered. However, the nugget 12 is quenched again. Thus, although the HAZ hardened portion 14H is tempered, the nugget 12 is quenched again, and hence the nugget 12 becomes hardened.
As a result, the toughness of the nugget 12 is deteriorated, and the peel strength of the spot-welded portion 10 is reduced.
As described above, with the conventional subsequent energization method, it is not easy to sufficiently obtain the effect of improving the toughness of the spot-welded portion, and there is a problem in which welding time increases, which leads to a notion that this conventional subsequent energization method is not practical. In order to solve these problems, various techniques have been disclosed.
Patent Document 1 discloses an invention in which conditions for subsequent energization are determined according to sheet sets through numerical calculation.
Patent Document 2 discloses an invention in which subsequent energization is applied at least once for a short period of time under high electric current conditions to effectively heat a portion that is to be a starting point of fracture, thereby reducing the welding time, and furthermore, the invention has a wide range of appropriate conditions.
Patent Document 3 discloses an invention that improves the fracture strength of the joined portion by increasing the width of the HAZ softening zone in the vicinity of the nugget through subsequent energization, and making the structure fine while maintaining the hardness of the nugget.
Patent Document 4 discloses an invention related to spot welding that can secure excellent tensile strength when applied to a high-tensile steel sheet, by forming the maximum point of hardness in a HAZ portion while maintaining the hardness of a nugget through spot welding with a simple two-step energization type formed by combining main energization and tempering energization.